In a typical radio frequency (RF) communications system, data may be processed at a transmitter device, modulated, filtered, amplified, and transmitted to one or more receiver devices. The transmitted signal typically experiences path loss, multipath interference, and other types of signal degradation before reaching the receiver devices. A receiver device receives the modulated signal, performs various types of conditioning, and demodulates the conditioned signal to recover the transmitted data. The signal conditioning typically includes low noise amplification, filtering, automatic gain control, frequency down-conversion, sampling, quantization, and so on.
Modern communication systems employ a variety of different modulation techniques to transfer data. For example, phase shift keying (PSK) is one common technique used to modulate a carrier with a multi-valued digital signal whereby digital values are represented by predetermined phases of the carrier signal. An M-ary phase shift keying arrangement such as quaternary phase shift keying (QPSK) may use the phases π/4, 3π/4, 5π/4 and 7π/4 in a phase plane representation to represent symbols comprising pairs of digital values or dibits. A demodulator receiving the PSK signal generally includes an oscillator operating at the same frequency and phase as the transmitted signal. To properly distinguish between symbol phases in the PSK signal at a receiver, the phase of the carrier source in the demodulator is set to that of the received signal.
When a signal is transmitted over a radio channel such as in mobile radio or satellite systems, interference of the transmission medium often subject the signal to severe variations in phase and amplitude. Due to path loss, multipath interference, and the other types of interferences, the received signal level typically varies over a wide range of values. For example, in some systems the received signal level may vary in excess of 90 dB. Consequently, the communication receiving devices require automatic frequency and phase control to maintain synchronization between the received signal carrier and the demodulator oscillator. The phases of the received signals are maintained at acceptable levels by proper design of receiver circuits, such as by the use of phase error detection circuits. Currently available phase error detectors generally fall into two categories: decision-directed phase detectors and non-decision-aided phase detectors.